How do you display a tool tip for a custom modal operator?
Area.header_text_set

A call to area.header_text_set(text) sets the header text to text No argument area.header_text_set() and  it returns header to usual mode (menus et al).

Code below from Scripting &gt; Templates &gt; Python &gt; operator_modal_view3d.py 

    if event.type == 'MOUSEMOVE':
        self.offset = (self._initial_mouse - Vector((event.mouse_x, event.mouse_y, 0.0))) * 0.02
        self.execute(context)
        context.area.header_text_set("Offset %.4f %.4f %.4f" % tuple(self.offset))

    elif event.type == 'LEFTMOUSE':
        context.area.header_text_set()
        return {'FINISHED'}